1. Field of the Invention
This invention relates to thermal-optical converters which change optically due to a sensitive response to heat. 2. Description of the Prior Art
As materials which change optically (visually) due to their sensitive response to heat, several heat-sensitive materials have already been proposed and provided for practical uses. One of these materials is a uniform dispersion of a finely divided, colorless leuco dyestuff (opaque) and a phenolic compound in an aqueous solution of polyvinyl alcohol, which dispersion is being used as a heat-sensitive paper for a thermal printer. Another one of these materials is composed of a cholesteric organic compound or K.sub.2 Cu(SO.sub.4).sub.2 and the like which can develop a color by releasing water from crystals. The two heat-sensitive materials mentioned above have been widely used in respective applicable fields. However, both materials have advantages and disadvantages, which cause them to be insufficient as materials for satisfying the following functions or properties.
(a) The reversible optical conversions of the materials must depend on the presence or absence of heat. PA1 (b) One phase of the two optical conversion phases which are dependent on the presence or absence of heat must be a "transparent" phase. PA1 (c) The structure or composition of the enclosures containing the materials must be simple and stable over a long period of time, and such structure can be realized in forms varying from small to extra-large sizes. PA1 (d) The materials must not cause environmental pollution. PA1 (e) The converters must be inexpensively produced.